The present invention pertains to emulsion acrylic adhesives for use in marking films.
Marking films are recognized in the art as laminates of a self-supporting polymeric film facestock or backing, such as polyvinyl chloride (PVC), and a layer of a pressure-sensitive adhesive (PSA). Prior to application to a substrate, the exposed surface of the PSA of the laminate is in contact with and protected by either a release liner or the outer surface of the facestock, which is provided with a release coating. Marking films are manufactured in the form of large rolls of the laminate, then cut to size depending on the application. Applications range from small advertising decals to automotive pinstriping to large printed signs, truck panels, and the like. The films may include or take the form of letters, numbers, logos and other indicia.
There are many requirements for a laminate to be used as a marking film. A very important requirement is durability of the laminate of the polymeric film facestock and adhesive for a period of months or years. The laminate of the polymeric film facestock and adhesive must remain functional for an expected period of time, after which the laminate must have retained the ability to be cleanly removed from the substrate without leaving an adhesive residue. Removal typically occurs with destruction of the facestock, with the facestock no longer being reusable. Accordingly, the adhesives are typically regarded more as xe2x80x9cpermanentxe2x80x9d than as xe2x80x9cremovablexe2x80x9d. Clean removability refers to removal without leaving an adhesive residue on the substrate after a period of time. For a midlife marking film, this period is over one year, and typically five or more years, exposure to the elements, which can be predicted by weatherometer tests such as evaluation using Xenon arc aging in an Atlas Weather-o-meter, where 2000 hours exposure is the equivalent of five years.
Polyvinyl chloride films that have been used for marking film applications have a varied durability that depends on service life. Short life vinyl films are used for promotional sign applications where an expected service life is six months to one year. If plasticized, the vinyl is typically film-plasticized with a monomeric plasticizer. Cost is kept low by elimination of any primer or anchor and barrier layers. Medium and long life films are typically expected to have a useful life of up to about ten years, and normally contain UV and heat stabilizing components and polymeric plasticizers. A primer may be employed to enhance anchorage of the pressure-sensitive adhesive to the film, and a barrier layer may be employed to inhibit migration of plasticizer from the body of the film into the adhesive. Medium life films are generally calendered and have an expected service life of about one to ten years. Long life films are typically cast from solvent and have an expected service life of ten or more years. Vinyl films typically have a thickness of from about 1 to 5 mils, more typically about 2 to 3 mils.
Historically, the PSAs used for marking films typically have been solvent-based adhesives. PSAs are typically regarded as xe2x80x9cpermanentxe2x80x9d if, when an attempt is made to remove a laminate of the PSA and a paper facestock from a substrate, the facestock tears. With vinyl or other polymeric facestocks, attempts at removal result in deformation of the facestock to the extent that it cannot be reused. The preferred adhesives desirably have some degree of initial repositionability, when the laminate is first applied to the substrate. Repositionability of the laminate without deformation of the facestock ends as adhesion grows with time due to molecular flow of the adhesive, which conforms to the substrate. Typically, the bond to a substrate will increase as much as 100% from an initial peel strength of about 12.5 N/25 mm after 10 to 15 minutes dwell on the substrate to about 15 N/25 mm to 17.5 N/25 mm within 24 hours. The requirement that the adhesive provide a sufficient bond strength such that the facestock is not reusable is an important characteristic for inhibiting vandalism of applied signs and other indicia in the form of advertisements, labels, and the like.
Other properties must also be present. The PSA must have sufficient shear strength to resist flow under the load of a thick polymeric facestock, it must yield to shrinkage of the facestock without exposing the adhesive, and it must be resistant to adhesive shrinkage so as to avoid wrinkling of the facestock. For outdoor applications, the PSA must be water-resistant with minimal to no significant loss of adhesion. Examples of typical outdoor applications include uses where the PSA is provided as a marking film label on a container exposed to sea water under transoceanic shipping conditions, and as a sign on public or private transportation throughfares in climates of high humidity or rainfall. Where the PSA is bonded to a transparent substrate such as glass, it is desirable that the PSA be clear or transparent and resist water whitening.
Another important property of PSA""s used in marking films used as signs and labels on truck sidings and the like is an ability to survive a squeegeeing application process, where a substrate to be labeled is spray-washed with a surfactant solution, the marking film laminate is laid out on the clean, wet substrate, and after positioning the marking film the excess water is squeegeed away. This presents the problem of water whitening, the resistance to which is important, especially where the substrate is transparent. If water whitening occurs it must dissipate in a matter of a few hours.
Because of the high performance demands and requirements placed on these adhesives, particularly of water resistance, the adhesives currently used for marking film applications have been organic solvent -based. Organic solvents are known to be noxious to the senses and may be hazardous. In addition, they present environmental problems, all of which are avoided if the PSA is water-based, such as emulsion PSAs. Additionally, solvent-based PSAs tend to be more expensive than emulsion PSAs. Emulsion PSAs, however, are notoriously water-sensitive and whiten in some instances by mere application of a drop of water. While some emulsion copolymers, such as a 2-ethylhexyl acrylate-butyl acrylate-vinyl acetate emulsion copolymer, have been employed for general purpose, short life, marking film applications, there have not yet been proposed functional adhesives for medium and longer term marking film service applications.
The present invention is, therefore, directed to water-resistant, i.e., resistant to water-whitening and water deterioration, PSAs where the carrier is water, and which, when employed in marking film label applications, will cleanly remove from the substrate even after many years of exposure to the elements.
According to the present invention, emulsion acrylic copolymers useful as PSAs for marking films applications are provided. The copolymers are inherently tacky and are useful as pressure-sensitive adhesives that form a permanent bond to a substrate, but are removable from the substrate without leaving an adhesive residuexe2x80x94even after years of exposure to the elements. Nonlimiting examples of such substrates include metal, glass, and painted surfaces. The copolymers are advantageously prepared as high solids content emulsions.
In one embodiment, the invention is directed to an inherently tacky, pressure-sensitive adhesive composition useful for marking films, comprising polymer particles prepared by emulsion polymerization of at least one monomer mixture comprising:
1) at least one alkyl acrylate, the alkyl group of which has from about 4 to 12 carbon atoms,
2) at least one unsaturated carboxylic acid containing from about 3 to 5 carbon atoms, and
3) at least one hard monomer, in addition to the at least one unsaturated carboxylic acid, preferably at least one styrenic monomer;
wherein the particles have a mean diameter of about 300 nm or less, as determined by laser light scattering. Particularly preferred are emulsion copolymer particles having a mean diameter of from about 100 to 250 nm, more preferably from about 135 to 190 nm. It has been discovered that adhesive compositions comprising particles having a diameter in this range are especially resistant to water and solvents.
In another embodiment, the invention is directed to an inherently tacky, pressure-sensitive adhesive composition useful for marking films, comprising polymer particles prepared by emulsion polymerization of at least one monomer mixture comprising:
1) at least one alkyl acrylate, the alkyl group of which has from about 4 to 12 carbon atoms, present in a total amount of from about 73% to 90% by weight, based on the total weight of the monomers;
2) at least one unsaturated carboxylic acid containing from about 3 to 5 carbon atoms, present in a total amount of from about 2% to 12% by weight, based on the total weight of the monomers, and
3) at least one hard monomer, other than the at least one unsaturated carboxylic acid, present in a total amount of from about 5% to 15% by weight, based on the total weight of the monomers.
In yet another embodiment, the invention is directed to an inherently tacky, pressure-sensitive adhesive composition useful for marking films, comprising:
a) a first copolymer formed by emulsion polymerization of a first monomer charge comprising:
1) at least one alkyl acrylate, the alkyl group of which has from about 4 to 12 carbon atoms, present in an amount of from about 73% to 90% by weight, based on the total weight of the first monomer charge;
2) at least one unsaturated carboxylic acid containing from about 3 to 5 carbon atoms and present in a total amount of from about 2 to about 12% by weight, based on the total weight of the first monomer charge, and
3) at least one hard monomer, other than the at least one unsaturated carboxylic acid, present in an amount of from about 5% to 15% based on the total weight of the first monomer charge; and
b) a second copolymer formed by emulsion polymerization of a second monomer charge comprising:
1) at least one alkyl acrylate, the alkyl group of which has from about 4 to 12 carbon atoms, and
2) at least one unsaturated carboxylic acid containing from about 3 to 5 carbon atoms and present in a total amount from about 7 to about 10% by weight, based on the total weight of the second monomer charge.
In the above embodiments, preferably one or more of the monomer mixtures further comprises at least one chain transfer agent.
Additionally, the above compositions preferably further comprise a surfactant system comprising at least one, and more preferably at least two, surfactants. Preferably, the surfactant system is present in the compositions in an amount of from about 0.5 to 5 parts by weight per 100 parts per weight of the monomers.
The compositions also can be crosslinked with at least one crosslinking agent. The crosslinking agent can be copolymerized with one or more of the monomer mixtures or added after polymerization of the monomer mixture(s).
Although not required, the compositions can be prepared by sequential polymerization of two or more monomer mixtures, as described above, whereby a first monomer mixture (or monomer charge) is allowed to react and then a second monomer mixture (or monomer charge) is allowed to react. The compositions of the two monomer charges may be the same or different. The first monomer charge is emulsion polymerized to form a first copolymer, and the second monomer charge is emulsion polymerized to form a second copolymer. In a preferred embodiment, the compositions of the monomer charges are the same, except that a chain transfer agent is added to the second monomer charge, resulting in the second copolymer having a lower molecular weight than the first copolymer.
Although not bound by theory, emulsion polymers formed by sequential polymerization are believed to have a core/shell structure, with copolymers formed from the first monomer charge making up the core of each emulsified particle, and copolymers formed from the second monomer charge making up the shell of each particle.